I am in the middle of the project now. I started by buying the 3rd magnet playfield so in a way I was committed a long time ago when I purchased it.
There are a lot of pieces to this puzzle. Unless you can procure the items cheaply, I have found it to be pretty damn expensive for the marginal gameplay differences. Yes it is cool when three balls are frozen, but it is a very rare occurrence when playing.
You need to do some serious electrical work as well. Plus from what I have read you take on the risk of messing up your other boards. If I could do it over again, I would skip it. Just my opinion.
For what it is worth, here is a link to a spreadsheet with notes on the restore. On it is a 3rd magnet tab with info on what and where I got stuff. I did already have the 3rd magnet kit which had the optos and the electronics for the high power board.